This application concerns the field of digital television, specifically, the construction of High Definition television frames so that four Standard Definition frames may be extracted from the data in one High Definition frame.
In the United States a standard, the Advanced Television System Committee (ATSC) standard defines digital encoding of high-definition television (HDTV) signals. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The standard is described in an International Standard publication entitled, “Information Technology-Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626,” ISO/IEC 13818-2, IS, 11/94 which is available from the ISO. The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image.
For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level is intended for coding video signals conforming to existing television standards (e.g., NTSC and PAL). Another standard, known as Main Profile, High Level is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line. The standard for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. The maximum data rate for the Main Profile, High Level standard is far greater than the maximum data rate for the Main Profile, Main Level standard.
In order for a digital HDTV video signal to fit in the transmission bandwidth of standard systems (i.e., systems conforming to National Television Systems Committee (NTSC) or Phase Alternate Line (PAL) standards), the HDTV video signal may typically be compressed using a ratio of about 5:1. For example, the NTSC standard requires an aspect ratio (ratio of width to height) of 4:3 with 525 scanning lines per picture and a television signal bandwidth of 6 MHz. The PAL standard specifies 625 scanning lines per picture and a television signal bandwidth of 8 MHz. Standard Definition television resolution video formats are typically 704-pixel-by-480-line or 640-pixel-by-480-line.
In contrast, HDTV-resolution video data typically requires 30 MHz of bandwidth, doubling the number of scan lines in a frame and changing the aspect ratio to 16:9. HDTV-resolution video is typically 1920-pixel-by-1080-line, although it may be in other formats, such as 1280-pixel-by-720-line in progressive or interlaced formats. High Definition (HD) video data may include any data at a resolution higher than Standard Definition (SD) video data, such as, for example, data with a resolution greater than 525 scan lines having a component rate of more than 30 frames per second, with 8 or 10-bit precision. HD data may be interlaced or progressively scanned and the present invention is not limited to either format.
The MPEG-2 standard defines a complex syntax that contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats to be covered by the standard. These formats define images having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or frame and differing numbers of frames or frames per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG-2 bit stream representing a sequence of images in five layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, and the macroblock layer. Each of these layers is introduced with control information. The present disclosure assumes the existence of a device capable of decoding encoded HDTV data into digital video. Such devices exist and are well known in the art.
At the present, Standard Definition video formats still dominate the video industries. There is a need for a method that uses an HD MPEG decoder as multiple independent SD MPEG decoders to decode Main Profile at Main Level MPEG streams and extract the independent SD video frames.